The use of ultra-wideband (UWB) technology is becoming increasingly popular in wireless communication systems. Radio systems that employ UWB technology have very wide operating bandwidth. This means that a much wider operating frequency range is advantageously available to UWB radio systems than conventional narrow-band radio systems. This distinctive feature of the UWB radio systems has prompted the US Federal Communication Commission (FCC) to regulate the operating frequency range of the UWB radio systems to between 3.1 and 10.6 GHz, with an effective isotropic radiated power (EIRP) not exceeding ˜41.3 dBm/MHz. The regulation limits the radiated power levels and signal spectra of the UWB radio systems in order to avoid interference to the conventional narrow-band radio systems which occupy a part of the frequency spectrum of the UWB radio systems.
Antennas for UWB radio systems need to be designed to fulfill a number of additional requirements. Firstly, the antennas need to have a bandwidth that is as broad and well-matched as possible for achieving broadband capability and attaining high radiation efficiency. Secondly, the antennas need to have a linear phase response for minimising distortion of signals which are transmitted through the antennas. Thirdly, the antennas need to radiate signals with maximum power in a desired direction.
With advancement in circuit integration and functionality, modern wireless communication devices, such as portable UWB DVD player and sensors, have become dimensionally smaller. The dimensions of the antennas have consequently become proportionally larger when compared to the overall dimensions of the UWB radio systems. Therefore, in conjunction with the abovementioned requirements for the UWB radio systems, a fourth requirement for designing UWB antennas is to reduce the dimensions of the antennas while still satisfying the other three requirements.
Numerous attempts have been made to fulfill the four requirements through various designs of antennas for the UWB radio system. More notable examples are transverse electromagnetic mode (TEM) horns and self-supplemental antennas, such as spiral antennas. Both types of antennas feature very broad and well-matched bandwidths. However, signals generated by both types of antennas are distorted and suffer from dispersion due to frequency-dependant changes in their respective phase centers.
Bi-conical and disk-conical antennas have less distortion and have relatively stable phase centers for achieving a broad and well-matched bandwidth. This is because resistive loadings are used to eliminate reflection of radiated pulses occurring at transmission ends of both antennas. However, both antennas are bulky in size and are thus unsuitable for the portable UWB devices.
Further attempts have been made to reduce the dimensions of UWB antennas by forming the antennas on printed circuit boards (PCBs). These attempts, however, have produced antennas which require a large ground plane for operation. The use of the large ground plane causes the operation of the antennas to be susceptible to changes in grounding conditions. This can substantially affect the operational stability of the antennas.
In U.S. Pat. No. 6,512,488, Schantz proposes a planar monopole antenna having a circular shape. The monopole antenna forms a parasitic open-grounded loop during operation to achieve broadband characteristics. However, the monopole antenna requires a ground plane for which operational stability can be substantially affected by changes to grounding conditions.
In U.S. Pat. Nos. 5,627,550 and 5,680,144, planar antennas having rectangular and triangular notches are proposed by Sanad for size reduction. However, the planar antennas are similarly susceptible to variable grounding conditions and the bandwidth of the monopole is also not sufficiently broad for UWB applications.
There is therefore a need for a UWB antenna which is dimensionally small and substantially independent of grounding conditions for use in small portable UWB devices.